The invention relates generally to combined cycle power systems and more specifically to an apparatus and method for starting a steam turbine against rated pressure.
As is known, combined cycle power systems include one or more gas turbines and heat recovery steam generators (HRSG's) and a steam turbine. Traditional combined cycle system startup procedures include low load holds of the gas turbine and restrictions on the gas turbine loading rate to control the rate of increase in steam temperature. These holds and restrictions contribute to air emissions during the startup event, may increase starting and loading time, and may increase fuel consumption during starting and loading.
More specifically, with combined cycle systems during starting and loading, and prior to the gas turbine achieving full load, the gas turbine is put on a hold until the temperature of the steam generated by the HRSG matches the steam turbine high pressure and intermediate pressure bowl metal temperature and/or the HRSG warms at an allowable rate and/or the HRSG is warmed up to the point of being ready for fuel heating. By holding the gas turbine at low load, the gas turbine operates at a low efficiency and with high exhaust emissions.
Such traditional starting procedures have been tolerated at least in part because in the past, startups were infrequent. With day to night power price swings, however, such startups have become more frequent.
In U.S. Pub. 2007/0113562 Tomlinson et al. (assigned to General Electric Co.) described methods and apparatus to facilitate reduced emissions during starting and loading with respect to emissions with known, traditional combined cycle systems. Such methods and apparatus also facilitated reduced starting and loading time and reduced fuel consumption during the starting and loading event as compared to known, traditional combined cycle systems.
The method includes loading the gas turbine at up to it's maximum rate, and loading the steam turbine at its maximum rate with excess steam bypassed to the condenser while maintaining the temperature of steam supplied to the steam turbine at a substantially constant temperature from initial steam admission into the steam turbine until all steam generated by the heat recovery steam generator is being admitted to the steam turbine while the gas turbine operates at up to maximum load.
FIG. 1 is a schematic illustration of a combined cycle power system 10 adapted for implementing a fast startup. As is known, system 10 includes a gas turbine 12 and a steam turbine 14 coupled to a generator 16. Steam turbine 14 is connected by multiple conduits to a heat recovery steam generator (HRSG) 18 and its exhaust is connected to a condenser 20.
The system 10 includes attemperators 22 at the discharge terminal of the high pressure superheater and attemperator 24 located at the discharge terminal of the reheater in HRSG 18. HRSG 18 may have a once-through or a drum type evaporator, which is capable of tolerating daily startup and loading of gas turbine 12 at an optimized rate with normal life span and maintenance.
During startup and loading the gas turbine and steam turbine, attemperators 22 and 24 operate to reduce the temperature of high pressure and hot reheat steam generated by HRSG 18 that is supplied to steam turbine 14. Particularly, attemperator 22 facilitates satisfying steam turbine criteria for steam temperature to high pressure bowl metal temperature matching with gas turbine 12 at any load. The temperature of the hot reheat steam for admission to the steam turbine intermediate pressure section is controlled to the steam to metal temperature matching criteria by the reheat steam terminal attemperator 24.
System 10 further includes bypass paths 28, 30 and 32 from HRSG 18 to condenser 20 and bypass path 33 from the high pressure steam line to the cold reheat steam piping that provide alternate high pressure steam flow paths while the steam turbine admission valves 40 are modulated to load the steam turbine at its fastest allowable rate. Bypass paths 28 and 33 include valves that are modulated to control the pressure of the high pressure steam and the rate of increase of high pressure steam pressure. Bypass path 30 provides an alternate path for the hot reheat steam while the intermediate pressure control valve is modulated during steam turbine loading. Bypass path 30 includes a valve that is modulated to control the reheat steam pressure while the steam turbine intermediate pressure control valve is modulated during steam turbine loading. Steam bypass path 32 provides an alternate path for the low pressure steam while the steam turbine low pressure admission valve is modulated during steam turbine loading. This bypass arrangement allows for 100% or greater steam generation by HRSG 18 with gas turbine 12 at up to maximum load with steam the turbine at any load from no load to a maximum load.
In addition, a steam turbine loading procedure is utilized that holds constant steam temperature from initial steam admission until all of the steam generated by the HRSG with gas turbine 12 at up to maximum load is being admitted and steam turbine loading can be performed at any gas turbine load up to maximum load. This maybe accomplished by maintaining the setpoint temperature of the high pressure steam terminal attemperator 22 at either the lowest allowable temperature (for example at approximately 700 F.) or if the bowl metal temperature is higher than the minimum, slightly above the measured temperature of the steam turbine high pressure bowl metal temperature when high pressure steam is initially admitted to the steam turbine. Likewise, the hot reheat steam terminal attemperator 24 setpoint is maintained at either the lowest allowable temperature or if the steam turbine intermediate pressure bowl metal temperature is above the minimum when steam admission is initiated, at a temperature at or slightly above the bowl metal temperature. This startup procedure facilitates steam turbine loading while facilitating minimum stress resulting from turbine shell or rotor heating.
Main steam control valve 40 is provided to steam turbine 14 for control of flow to the turbine. Main steam stop valve 41 is provided for positive isolation of steam to the turbine and quick closing for steam turbine protection. After all steam flow is being admitted to steam turbine 14, the steam temperature is raised at a rate compatible with allowable steam turbine stress and differential expansion to achieve normal steam turbine output and efficiency. Main steam control valve 40 and main steam stop valve 41 may be incorporated within a single body. Terminal attemperators 22 and 24 provide control of steam temperature during this steam turbine startup.
Traditionally, main stop and control valves for combined cycle applications have been designed for to the following requirements: 1) fast closure for turbine protection from overspeed; 2) low pressure drop for optimized turbine output and performance; and 3) light duty short duration throttling associated with starting up the steam turbine. Therefore, throttling requirements were relatively benign.
For implementation of rapid startup profile of Tomlinson et al. the main control valve will be required to perform heavy duty throttling against full rated upstream pressure. Past experience on applications with less severe, but still significant throttling for combined cycle main steam control valves (MSCVs) has been less than favorable. Both the reliability and low load controllability of these high performing MSCV's have been issues.
Accordingly, there is a need to provide throttling capability for severe pressure drops while maintaining low load control for steam to the steam turbine. Further, such an apparatus must maintain rapid closure for turbine protection and low pressure drop for turbine performance.